Concentration critical micelle

The critical micelle concentration (cmc) denotes the surfactant concentration range at which surfactant molecules associate to form micelles. The term cmc has been defined as the surfactant concentration in a solution in which just half of the total surfactant present is in the monomeric form [139]. Because several physical properties of micellar solutions change abruptly at cmc, the value of cmc and its dependence on surfactant structure, composition of the solution, and physical conditions are of great practical importance. Wetting, detergency, solubilization, and other important processes depend on the cmc of the micellar solution. 

The critical micelle concentration of a surfactant solution can be determined by measuring surface tension, electric conductivity, osmotic pressure, light scattering, viscosity, dye solubilization, and other physical properties [107] (see Chapter 9). The cmc in aqueous solution can be determined also from a change in the chemical environment indicated by the -NMR chemical shift (5) [140), plotted as a function of inverse surfactant concentration (Fig. 6.14). The intersection in the lines indicates the cmc. 

The cmc values for fluorinated A alkylpyridinium chlorides have been obtained from osmotic coefficients determined by a vapor pressure osmometer [141]. A critical discussion of the methods for the determination of cmc values has been published by Mukerjee and Mysels [142]. 

The cmc of fluorinated or hydrocarbon surfactants decreases exponentially with increasing number of carbon atoms, according to the empirical equation [143-145) 

In analogy to surfactants with a hydrocarbon chain, the critical micelle concentrations for fluorinated surfactants in an aqueous solution depend on the fluorocarbon chain length and the counterion (Table 6.9) [58,59,75,77,96,139, 146-154] and decrease with increasing hydrophobe chain length. 

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The concentration at which micellization commences is called the critical micelle concentration, erne. Any experimental teclmique sensitive to a solution property modified by micellization or sensitive to some probe (molecule or ion) property modified by micellization is generally adequate to quantitatively estimate the onset of micellization. The detennination of erne is usually done by plotting the experimentally measured property or response as a hmction of the logarithm of the surfactant concentration. The intersection of asymptotes fitted to the experimental data or as a breakpoint in the experimental data denotes the erne. A partial listing of experimental... 

The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

Mukeqee P and Mysels K J 1970 Critical Micelle Concentrations of Aqueous Surfactant Systems (National Standard Reference Data System, National Bureau of Standards Circular No 36) (Springfield, VA National Teehnieal Information Serviee)... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Dominguez, A. Fernandez, A. Gonzalez, N. et al. Determination of Critical Micelle Concentration of Some Surfactants by Three Techniques, /. Chem. Educ. 1997, 74, 1227-1231. 

At low concentrations surfactant molecules adsorbed at the surface are in equilibrium with other molecules in solution. Above a threshold concentration, called the critical micelle concentration (cmc, for short), another equilibrium must be considered. This additional equilibrium is that between individual molecules in solution and clusters of emulsifier molecules known as micelles. 

During Stage I the number of polymer particles range from 10 to 10 per mL. As the particles grow they adsorb more emulsifier and eventually reduce the soap concentration below its critical micelle concentration (CMC). Once below the CMC, the micelles disappear and emulsifier is distributed between the growing polymer particles, monomer droplets, and aqueous phase. 

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

One reason for widespread interest in the use of surfactants as gas mobihty control agents is the effectiveness at concentrations of <0.1 wt % (156,163). Some surfactants are effective below their critical micelle concentration (164). This low chemical requirement can significantly improve process economics. 

Ahphatic amine oxides behave as typical surfactants in aqueous solutions. Below the critical micelle concentration (CMC), dimethyl dodecyl amine oxide exists as single molecules. Above this concentration micellar (spherical) aggregates predorninate in solution. Ahphatic amine oxides are similar to other typical nonionic surfactants in that their CMC decreases with increasing temperature. 

Fig. 3. Schematic diagram of anionic surfactant solution at equiUbrium above its critical micelle concentration, where M = micelle and 0 are counterions ...

Critical Micelle Concentration. The rate at which the properties of surfactant solutions vary with concentration changes at the concentration where micelle formation starts. Surface and interfacial tension, equivalent conductance (50), dye solubilization (51), iodine solubilization (52), and refractive index (53) are properties commonly used as the basis for methods of CMC determination. 

P. Mukerjee and M. J. Mysels, Critical Micelle Concentration ofMqueous Suf actant Systems, NSROS-NBS 36, U.S. Dept, of Commerce, Washiagton, D.C., 1971. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

Surfactant values are at the critical micelle concentration (CMC) in aqueous solution surfactant/defoamer values are at 0.1% concentration in aqueous solution. 

MeutralSoluble Salts. So dium sulfate [7757-82-6] and, to a considerably lesser extent, sodium chloride [7647-14-5] are the principal neutral soluble salts used in laundering compositions. They are often considered to be fillers although they perform an important standardizing function enabling the formulator to manufacture powders of a desired, controlled density. Sodium sulfate, in addition, lowers the critical micelle concentration of organic surfactants and thus the concentration at which effective washing can be achieved. 

Surfactants lower the surface tension of water, typically from 72 to ca 30—35 mN/m (= dyn/cm), and many surfactants have a strong effect on the contact angle when used at low concentrations. Both changes help dewatering. Too much surfactant, near or above the critical micelle concentration... 

Finally, some general rules for the amount of surfactant appear to be vaHd (13). For anionic surfactants the average size of droplets is reduced for an increase of surfactant concentration up to the critical micellization concentration, whereas for nonionic surfactants a reduction occurs also for concentrations in excess of this value. The latter case may reflect the solubiHty of the nonionic surfactant in both phases, causing a reduction of interfacial tension at higher concentrations, or may reflect the stabilizing action of the micelles per se. 

When apphed to a nonionic surfactant in pure water at concentrations below the critical micelle concentration, Eq. (22-42) simplifies into Eq. (22-43)... 

K is K, just below the collectors critical micelle concentration, C,. Ko is Ki at some higher cohector concentration, C,. E is the relative effectiveness, in adsorbing cohigend, of surface cohector versus micehar collector. Generally, E > 1. F, is the surface excess of collector. More about each K is avahable [Lemhch, Adsubble Methods, in Li (ed.). Recent Developments in Separation Science, vol. 1, CRC Press, Cleveland, 1972, pp. 113-127 Jashnani and Lemlich, Ind. Eng. Chem. Process Des. Dev., 12, 312 (1973)]. 

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